possess them to survive and reproduce. Tigers have sharp teeth because tigers with sharp teeth do better than tigers with dull teeth. The reason the trait evolved is not that sharp teeth help the species to avoid extinction or somehow keep the ecosystem from collapsing. In examples of this sort, Darwin embraces what biologists now call “individual” selection. An exception to this pattern of thinking occurs when Darwin considers the evolution of human morality. Why do human beings often sacrifice their welfare for the good of the group? This is how Darwin sets the problem in the Descent of Man:
It is extremely doubtful whether the offspring of the more sympathetic and benevolent parents, or of those which were the most faithful to their comrades, would be reared in greater number than the children of selfish and treacherous parents of the same tribe. He who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature. The bravest men, who were always willing to come to the front in war, and who freely risked their lives for others, would on an average perish in larger number than other men.
Darwin (1871, p. 163)
Then he proposes his solution:
It must not be forgotten that although a high standard of morality gives but a slight or no advantage to each individual man and his children over the other men of the same tribe, yet that an increase in the number of well-endowed men and an advancement in the standard of morality will certainly give an immense advantage to one tribe over another. A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection. At all times throughout the world tribes have supplanted other tribes; and as morality is one important element in their success, the standard of morality and the number of well-endowed men will thus everywhere tend to rise and increase.
Darwin (1871, p. 166)
Here, Darwin invokes the hypothesis of group selection. When groups compete, characteristics that are deleterious to the individuals who have them can evolve because they are good for the group in which they occur. Biologists now call such traits “altruistic.” For Darwin, natural selection can involve both individual and group selection.
Darwin discusses 2 examples of altruism in the Origin—the barbed stinger of honeybees and the sterility of workers found in many species